In recent years, mobile communication terminals such as a portable telephone and a mobile telephone have been widely used, and various kinds of multiple access methods have been developed for use in such mobile communication systems. Among them, a CDMA (Code Division Multiple Access) method has been employed in portable telephones and others because it has high quality reception capability through the exploitation of multipath fading, and can achieve a high utilization efficiency of radio resource (can increase a subscriber capacity). A state of communication to a base station may be impaired due to, e.g., movement of a mobile station such as a portable telephone employing the CDMA system. In this case, so-called hand-over, which is the operation of switching to the communication channel of another base station, is performed for maintaining a better communication state.
FIG. 13 shows occurrence of the hand-over. In general, a plurality of base stations (BS1–BS5) are arranged regularly, and cells of the base stations form a regular polygon if these base stations are arranged to cover a service area with as high an electric field as possible, as is well known and shown in FIG. 13. When a mobile communication terminal (MS) performs the communication, it receives a plurality of radio waves from each base station. When the mobile communication terminal moves through a boundary of an electric field polygon, so-called cells or sectors, the mobile communication terminal and MS exchanges the access channel to keep communication. That is called hand-over. The hand-over is executed based on the MS's measurement of each cell, and a maximum level channel is usually used. When fading occurs, the channel at the maximum level frequently changes so that the hand-over is unexpectedly performed.
For the mobile communication terminal, such a method is generally known that changes the timing for sampling clock for performing timing tracking. According to the timing control for the sampling clock by a digital circuit, however, the accuracy of the change becomes low. In the case where the foregoing hand-over is frequently performed, therefore, the quality of data receiving operation deteriorates.
The invention has been developed for overcoming the above problem, and a first object of the invention is to provide a mobile communication terminal, which can prevent lowering of a receiving quality due to frequent switching of a most significant cell/sector.
A second object of the invention is to provide a mobile communication terminal, which can prevent lowering of a receiving quality due to frequent switching of a primary path.
A third object of the invention is to provide a mobile communication terminal, which can produce sampling clocks of different timing by a simple structure.